Electrowetting is a microfluidic phenomenon that modifies the shape of a liquid in relation to a surface by applying an electrical field, e.g. by applying a voltage across two electrodes. For example, if the surface is hydrophobic, the electrical field causes a change in the shape of the liquid that appears to change the wetting properties of the hydrophobic surface. If the fluid(s) in an electrowetting cell and some of the wall(s) around the fluid(s) are sufficiently transparent with respect to a light wavelength range of interest, the electrowetting cell may be used as an electrically controllable optic. Such optics have recently been the subject of a widening scope of light processing applications, such as variable lenses, variable prisms, optical switches, displays, etc.
Electrowetting lenses, for example, are conventionally used in the camera industry. These lenses tend to be very small (e.g. millimeter scale) and operate in a small tunable range (small range of input or output light angle). The thickness of the fluidic lenses are also typically less than half the cell size. Such small effective lens sizes tends to limit the functionality of any given structural design of the electrowetting optic. An electrowetting cell structure for a lens for a camera application or the like, e.g. to selectively focus light input to an image sensor or to selectively control beam distribution of a flash, typically supports only beam shaping.
There have been proposals to develop variable optical prisms using electrowetting cell arrangements. An electrowetting lens may have various different shaped structures, e.g. round, square or rectangular. An electrowetting prism normally is square or rectangular. The overall working principle for either beam shaping or steering is the same—the voltage applied across the dielectric layer attracts or repels the conducting liquid so as to change the wetting area of the cell and thus the shape of the liquid(s) in the cell.
Typically, individual electrowetting cells have been small, for example several millimeters across (the diameter or diagonal of) the active optical area of the cells. Adaptations of such cells for larger scale light processing applications requires combining a number of such small cells into a larger area array or matrix, which increases manufacturing complexity and cost and may increase the complexity of the circuitry needed to drive the array of cells. As size of the cell increases, one of the negative effects relates to the impact of external forces, such as gravity, on the shape of the meniscus between the two fluids and thus the shape of the optical lens or prism provided by the cell. The impact of an external force also is directional, in that the resulting distortion of the meniscus depends on the orientation of the cell relative to the direction of the applied force.